1. Field of the Invention
The present invention relates to a band elimination filter used in a communication apparatus, such as a cellular phone and car phone.
2. Related Art of the Invention
In the prior art a surface acoustic wave filter or piezoelectric filter has been used as an RF filter in communication apparatus. The surface acoustic wave filters mainly used include a longitudinal coupled mode filter having a plurality of interdigital transducer electrodes (IDT electrodes) closely arranged in the propagation direction and a ladder filter having surface acoustic wave resonators interconnected in a ladder-like arrangement. On the other hand, as the piezoelectric filter, a bulk wave filter is used. It has been desired that these filters are increased in performance and reduced in size.
In the following, a conventional band elimination filter will be described with reference to the drawings.
FIG. 19(a) shows a configuration of a surface acoustic wave resonator. In this drawing, the surface acoustic wave resonator comprises an IDT electrode 1702 formed on the piezoelectric substrate 1701, reflector electrodes 1703 and 1704.
FIG. 19(b) shows a configuration of a piezoelectric resonator. In this drawing, the piezoelectric resonator comprises a piezoelectric layer 3011, an upper electrode 3012 formed on an upper principal plane of the piezoelectric layer 3011, a lower electrode 3013 formed on a lower principal plane of the piezoelectric layer 3011 and a substrate 3014. There is formed a depression in the surface of the substrate 3014 which is in contact with the lower electrode 3013, and the depression constitutes a cavity 3015. In this configuration, the upper electrode 3012, the lower electrode 3013, the piezoelectric layer 3011 sandwiched between the upper electrode 3012 and the lower electrode 3013, and the part of the substrate 3014 which constitutes the cavity 3015 constitute the piezoelectric resonator.
The surface acoustic wave resonator and the piezoelectric resonator are each represented by an equivalent circuit shown in FIG. 19(c), and have electric characteristics that provide serial resonance and parallel resonance, respectively.
Connecting a plurality of such surface acoustic wave resonators in a ladder arrangement provides a ladder surface acoustic wave filter (for example, see Japanese Patent No. 3152418, the disclosure of which is incorporated herein by reference in its entirety).
Such a conventional acoustic resonator will now be described by taking a surface acoustic wave resonator as an example.
FIG. 20(a) shows, as a conventional example 1, a configuration of a surface acoustic wave filter formed by interconnecting three surface acoustic wave resonators 1801, 1802 and 1803 in a π arrangement. As shown in FIG. 20(a), the surface acoustic wave resonators 1801 and 1802 each have one end grounded, and the other ends of the surface acoustic wave resonators are coupled to a transmission line at a predetermined interval, the transmission line 1804 having signal input and output terminals. The surface acoustic wave resonator 1803 is disposed in the predetermined interval on the transmission line 1804.
In this configuration, as for the passing characteristics, the pass band and the attenuation band of the filter depend on the resonance and anti resonance frequencies of the parallel surface acoustic wave resonators 1802 and 1803, which are placed in parallel with the serial surface acoustic wave resonator 1801 as shown in FIG. 20(b). However, there cannot be provided a band elimination filter having a low loss over a wide band.
FIG. 21(a) shows, as a conventional example 2, a circuit of a band elimination filter formed by connecting two surface acoustic wave resonators 1901 and 1902 in parallel. As shown in FIG. 21(a), the surface acoustic wave resonators 1901 and 1902 each have one end grounded, and the other ends of the surface acoustic wave resonators are coupled to a transmission line 1903 at a predetermined interval, the transmission line 1903 having signal input and output terminals. As shown in FIG. 21(b), although the filter has a low loss at frequencies higher than the stop band (attenuation pole), it has a high loss at frequencies lower than the stop band (attenuation pole).
As described above, with the surface acoustic wave filter composed of a plurality of acoustic resonators, such as surface acoustic wave resonators, used in communication apparatus or the like, it has been difficult to provide characteristics of a high attenuation within a desired frequency range and a low loss over wide frequency bands lower and higher than a stop band.
The present invention has been devised in view of the problem described above. An object of the invention is to provide a band elimination filter or the like that provides characteristics of a high attenuation within a desired frequency band and a low loss over wide frequency bands lower and higher than a stop band.